Radioautographic methods at the ultrastructural level will be used to study synaptogenesis by the regenerating optic fibers in the goldfish optic tectum. By 10 days after optic tract section, regenerating fibers have begun to enter the tectum and by 3 to 4 weeks post-operatively, visual function has returned. During this period, significant synaptogenesis occurs. Highly concentrated tritiated amino acids will be injected into the eye contralateral to the sectioned tract and 18 hours later, the animals will be perfused and prepared for EM and radioautography. The regenerated fibers and boutons containing transported, labelled protein can then be visualized and the sequence of morphological events can then be described. The process of regeneration is not complete with the return of function. The further elaboration of retinal projections to the tectum and elsewhere in the brain will be determined using radioautographic methods at the light level at intervals of 1 to 6 months postoperatively. If regeneration is prevented, the conditions required for collateral sprouting are present, i.e., denervated postsynaptic spaces. Enucleation prevents regeneration and preliminary evidence suggests that some degree of sprouting may occur in the tectum. The possibility that axons which do not arise from the contralateral retina may be capable of making synapses on denervated tectal neurons implies that some form of competition normally exists between axonal systems. We will begin to define the properties which determine the successful reinnervation of tectal cells by comparing the ultrastructural events occuring in the denervated tectum with those occuring in the regenerating system.